This is Steven Salzberg's blog on genomics, pseudoscience, medical breakthroughs, higher education, and other topics, including skepticism about unscientific medical practices. Here's where I can say what I really think about abuses and distortions of science, wherever I see them.

How much brain damage is too much? NFL players head for the exits.

Not because he was unable to play, and certainly not because of his age–he's only 26. No, Baltimore Ravens' player John Urschel decided to quit because the risk of permanent, irreversible brain damage is just not worth it.

Urschel is a very smart guy. He's currently pursuing a Ph.D. in mathematics at MIT, one of the best and most demanding science universities in the world. Until this summer, he was (impressively) balancing his studies with being a full-time NFL player.

Let's get one point out of the way: everyone involved with the study, including Dr. McKee, knows that it was biased. The scientists examined brains of deceased players that had been donated to the study because family members–or the players themselves, before they died–suspected something was wrong. So perhaps the true risk of brain damage is lower than 99%. Maybe it's only 50%, or 20%. Do young men playing football want to take that risk?

The NFL has been denying or downplaying the risk for years. A few years ago, after the suicide of former player Junior Seau, they announced a $30 million partnership with the NIH to study the risks of football on the brain. As results started coming in, showing that the risk was far more serious than most people knew, the NFL backed out of the deal with $16 million still unspent.

"unless there is a way to reduce the number of TBIs [traumatic brain injuries] caused by the sport, football will remain a threat to the brains and health futures of the players, including impaired cognitive function and reasoning, memory loss, emotional depression, and other sequelae that profoundly erode quality of life."

Earlier this year, a study out of the CDC reported that "3 high school or college football players die each year from traumatic brain and spinal cord injuries that occur on the field," most them as a result of being tackled during games.

Over the years, football players have grown ever larger (the average NFL lineman today weighs over 300 pounds) and the intensity of the violence on the field has grown with them. It's not just in the NFL, either: last year, three high school teams in the state of Washington forfeited their games against a local team out of a legitimate fear that players would be badly injured by the opposing team's 300-plus pound linemen. Their fears were justified: the human head simply wasn't built to withstand the repeated blows that players endure.

All players might do themselves a favor by listening to John Urschel. He explained his decision–and his abiding love for the game of football–in a lengthy interview on the Freakonomics podcast a couple of weeks ago. That interview should be required listening for young players, and even more so for parents who might be dreaming that their sons have a future career in football.